A power system, such as an electric grid, typically comprises a transmission network which transfers power from power generating stations, e.g. power plants, and a distribution network connected to the transmission network for distribution of the power to loads, such as households and factories connected to the distribution network.
The transmission network is typically based on a Flexible Alternating Current Transmission System (FACTS) and/or a High Voltage Direct Current (HVDC) system.
AC power transmission gives rise to electromagnetic fields resulting in reactive power components in the grid e.g. due to inductive and capacitive loads and the inductance of the power lines.
By decreasing the reactive power, the active power which acts to operate loads connected to the power system can be increased. At other times, it may be desirable to provide additional reactive power to the power system in order to stabilize it.
FACTS provide stabilization of a power system by means of reactive power compensating devices such as Static VAR units and STATCOM units.
Static VAR units such as Thyristor Controlled reactors (TCR) produce harmonics of the fundamental frequency in the power system. Harmonics are undesired by-products resulting from switching the thyristors. It is normally desirable to reduce the harmonic content produced by a TCR. Especially, it is desirable to prevent or at least reduce the harmonic content generated by a wye-connected TCR to be fed into the power system. Compared to a delta-connected TCR, the 3rd, 9th, . . . , (3n)th harmonic content generated by a wye-connected TCR is fed into the power system, whilst for a delta-connected TCR the 3rd, 9th, . . . , (3n)th harmonic current is essentially trapped as a circulating current in the delta-connection.
STATCOM units utilize voltage converters comprising chain-linked, i.e. series connected, converter cells having switchable semiconductor devices. By switching the semiconductor devices properly in the converter cells, the amount of reactive power in the grid can be controlled.
Some converter topologies utilise a DC-capacitor in each converter cell, e.g. H-bridge cells, in order to control the voltage generated by each converter cell. The DC voltage over each capacitor should typically be kept constant according to a respective set-point value both during normal operation of the power system and under asymmetrical conditions.
For delta-connected STATCOM units, a circulating current is trapped in the delta-connection, enabling voltage control of the DC-capacitors in the converter cells. However, for wye-connected STATCOM units, the current of the STATCOM may be zero. Voltage control of the DC-capacitors is lost under such conditions. Therefore, in order to control the voltage level of the DC capacitors, a current has to be generated and fed to the DC-capacitors. This current is fed into the power system.